Embodiments of the present invention relate to fluid cooled bearing systems, and particularly systems comprising an axial magnetic bearing cooled by a fluid flow.
Typically, in such a bearing, a fluid flow, for example a gas stream, is injected into one or more air gaps between a rotating flywheel of the axial bearing, and one or more fixed abutments of the axial bearing. The gas stream must be sufficient to evacuate both the calories generated by the magnetic induction phenomena, and the calories generated by the fluid viscous frictions at the air gap. Ventilation loss designates the latter type of loss.
When the bearing system is part of a rotating machine such as a turbine or a compressor, the high flow rates of gas necessary for cooling sometimes lead to establish a cooling fluid circuit configured to cool not only the bearing system, but also to cool other components of the machine.
This cooling circuit can be arranged internally to the machine, using, for example, the main fluid flowing through the rotating machine and the pressure differences existing naturally in this machine, or can be arranged separately by means of a dedicated cooling circuit possibly using another cooling fluid (for example, but not exclusively, air) and a generating system of dedicated fluid flow.
In all cases, the cooling flow rate directly generates an economic loss, either because of a yield loss of the turbomachine (internal system), or because of the cost linked to the investment and use of the external cooling system.
To reduce the cost of cooling, cooling fluid flow should be reduced while continuing to provide the same operating temperatures of the components of the bearing or of the bearing system.
The embodiments aim to propose a bearing, or a bearing system, cooled by a fluid circulation system which allows effective cooling of the bearing, i.e. which allows removing the calories generated by the magnetic, electrical, and ventilation losses, while using only a reduced flow rate of cooling fluid.
The embodiments propose to reduce the need for cooling by reducing ventilation losses.
Generally, the ventilation loss corresponds to the energy transferred by the rotating flywheel to the cooling fluid. This energy can be positive, negative or zero.
If the local speed of the flywheel is greater than the fluid speed, the flywheel causes the fluid to rotate. The fluid is heated. The greater is the difference in speed between the fluid and the flywheel, the more important the energy loss is.
If the local fluid speed is identical to that of the flywheel, there is no friction and no loss by ventilation.
If the fluid speed is greater than, and in the same direction as, the speed of the flywheel, the fluid drives the flywheel by transferring energy to it.
This suggests that the loss of ventilation can be limited or canceled by reducing the existing speed difference at any point between the stop wheel and the cooling fluid.
The general principle of embodiments of the invention is to inject the cooling fluid at high speed in the direction of rotation (tangential direction) of the stop in order to minimize friction losses. High speed means a tangential speed of fluid of 50% to 150% of the tangential speed of the stop at the point of the stop that passes in front of the injection point.
Simple considerations of conservation of angular momentum of the injected fluid show in particular that, for a given injection speed, it is more effective, for reducing the friction loss, to inject the fluid at the periphery of the flywheel rather than towards the internal diameter.